This invention relates to a wound rotor motor apparatus and particularly to an energy recovery and control circuit connected to the rotor and to the incoming power supply.
Induction motors of a wound rotor construction are advantageously used in various industrial applications. The wound rotor induction motor of a three-phase construction is particularly adapted to large torque and high power requirements. In such a three-phase motor, the stator includes a three-phase winding connected to a three-phase alternating current power supply. The rotor is wound with a three-phase winding which is interconnected to a rotor load circuit for absorbing the energy associated with the inherent slip characteristic of wound rotor motors. Resistance elements may be connected into the rotor load circuit to absorb and/or dissipate the energy. This results in a relatively inefficient operation but has been and is used in many installations, particularly with switching means to insert and remove resistance so as to adjust the motor characteristic. For example, cranes, hoists, elevators and like devices may advantageously use wound rotor motors with a changeable resistance rotor circuit. A more efficient wound rotor motor system has been suggested, particularly since the development of solid state circuit elements, wherein the rotor energy is coupled back to the power system and thus least partially recovered. The level of the power feedback from the rotor to the stator circuits is selected to control the motor speed. This results in a more effective and efficient motor system. For example, U.S. Pat. No. 3,136,937 discloses a basic operating system wherein a full wave diode bridge is connected to the three-phase rotor winding of a wound rotor induction motor. The direct current output of the diode bridge is connected as the input to a gated inverter bridge, the output of which is connected to the three-phase power supply system. The inverter includes gated thyristors in each leg of the bridge, with pair of thyristors fired from a firing control circuit to feed power back to the power line during appropriate periods of the alternating current phase voltages on the several phase lines. For efficient transfer of power and maintaining proper motor operation under desirable power factor conditions, the thyristors are gated on at a fixed time during the corresponding voltage cycle of the A.C. phase supply. A gated thyristor is connected across the diode rectifier and controlled to bypass a part of the energy from the inverter back through the rotor circuit to control the motor speed. A control system in combination with a stabilizing circuit across the inverter for absorbing transient voltages is disclosed in U.S. Pat. No. 3,504,254.
Various other systems have been developed with various stabilizing and control circuits. Generally, the prior art systems have included expensive, large line capacitors for power factor correction. Although such prior art developments have been suggested and energy recovery systems have been satisfactorily used, the prior art devices have not therefore provided optimum energy recovery.